generate bindings for Node.js

[venediktov]

Currently we provided simple general binging code to access C++14 bidder via main function
, however for ppl to write logic in JavaScript I think we need the following :

• Add --bindings flag to bid_generator.go
• New codegen/bindings_generator.go
• Generate Node.js C++ wrapper function arround C++ matching functions see generated bidder.cpp.
• Provide minimum example on bidder.js how to use those newly generated Node C++ functions in JavaScript. Because JavaScript layer does not yet have fwk/rtb/core/ad_selector.hpp with chained_selector class implementation, therefore developers will have to write javascript logic by hand, however chained_selector.js can be implemented , so the use case in bidder.js will be similar as in generated bidder.cpp

This is still Can we do it type of issue , needs evaluation from node.js ppl.